A Scientometric-Analysis-Based Review of the Research Development on Geopolymers

A scientometric-based assessment of the literature on geopolymers was conducted in this study to determine its critical aspects. Typical review studies are restricted in their capability to link disparate segments of the literature in a systematic and exact way. Knowledge mapping, co-citation, and co-occurrence are very difficult components of creative research. This study adopted an advanced strategy of data mining, data processing and analysis, visualization and presentation, and interpretation of the bibliographic data on geopolymers. The Scopus database was used to search for and retrieve the data needed to complete the study’s objectives. The relevant sources of publications, keyword assessment, productive authors based on publications and citations, top papers based on citations received, and areas actively engaged in the research of geopolymers are recognized during the data assessment. The VOSviewer (VOS: visualization of similarities) software application was employed to analyze the literature data comprising citation, bibliographic, abstract, keywords, funding, and other information from 7468 relevant publications. In addition, the applications and restrictions associated with the use of geopolymers in the construction sector are discussed, as well as possible solutions to overcome these restrictions. The scientometric analysis revealed that the leading publication source (journal) in terms of articles and citations is “Construction and building materials”; the mostly employed keywords are geopolymer, fly ash, and compressive strength; and the top active and contributing countries based on publications are China, India, and Australia. Because of the quantitative and graphical representation of participating nations and researchers, this study can help academics to create collaborative efforts and exchange creative ideas and approaches. In addition, this study concluded that the large-scale usage of geopolymer concrete is constrained by factors such as curing regime, activator solution scarcity and expense, efflorescence, and alkali–silica reaction. However, embracing the potential solutions outlined in this study might assist in boosting the building industry’s adoption of geopolymer concrete.


Introduction
Cement concrete is the most widely used material in construction [1][2][3][4], with annual worldwide consumption levels of over 30 billion tons [5][6][7], depleting significant amounts of ordinary Portland cement (OPC), the primary binder utilized to manufacture traditional concrete [8][9][10][11]. Nevertheless, the manufacture of OPC is linked to major environmental challenges, such as the intense consumption of natural resources and the generation of greenhouse gases [12][13][14][15]. Roughly 1.5 tons of raw ingredients are required to create a single ton of OPC, which generates approximately 0.55 tons of carbon dioxide directly, whilst the burning of fuels generates approximately 0.4 tons of carbon dioxide, for an overall of 0.8-1.0 tons of carbon-dioxide discharge [16]. The cement industry consumes a substantial quantity of fossil fuels, which accounts for around 12-15% of industrial with the most papers and citations, top articles in terms of citations, and the regions actively involved in the research of geopolymers. The Scopus search engine was used to obtain containing citations, bibliographic, abstract, keywords, funding, and other information from 7468 pertinent papers, which were then analyzed employing the VOSviewer application. As a result of the graphical and statistical representation of researchers and nations, this study will assist scholars in developing collaborative endeavors and exchanging innovative concepts and techniques. Following a scientometric analysis of subject-related keywords and a review of the most pertinent literature, this study highlighted and discussed the current-state applications of geopolymer concrete, the limitations associated with the production and use of geopolymer composites, and potential solutions. Advantages of geopolymer concrete made from waste materials [67].
As research on the geopolymers increases in response to growing environmental concerns associated with the use of ordinary cement concrete, scientists are confronted with information restrictions that may stifle innovative research and academic collaboration. Consequently, it is crucial to develop and implement a system that facilitates academics to acquire essential knowledge from as highly credible sources as possible. Using a software program, a scientometric technique may help overcome this deficiency. This project aims to conduct a scientometric study of bibliographic records published on geopolymers up to May 2022. A scientometric assessment can conduct a quantifiable assessment of an enormous amount of bibliographic data by utilizing the appropriate software application. Conventional review studies lack the capacity to accurately and comprehensively link disparate portions of the literature. Scientific visualization, co-citations, and co-occurrence are among the highly complicated aspects of contemporary research [11,47,67]. Scientometric analysis reveals the sources with the most publications, keyword co-occurrence, the authors with the most papers and citations, top articles in terms of citations, and the regions actively involved in the research of geopolymers. The Scopus search engine was used to obtain containing citations, bibliographic, abstract, keywords, funding, and other information from 7468 pertinent papers, which were then analyzed employing the VOSviewer application. As a result of the graphical and statistical representation of researchers and nations, this study will assist scholars in developing collaborative endeavors and exchanging innovative concepts and techniques. Following a scientometric analysis of subject-related keywords and a review of the most pertinent literature, this study highlighted and discussed the current-state applications of geopolymer concrete, the limitations associated with the production and use of geopolymer composites, and potential solutions.

Review Strategy
This study conducted a scientometric analysis of bibliographic data [68][69][70] in order to quantify the numerous characteristics of bibliographic data. Scientometric studies utilize scientific mapping, a technique established by academics for bibliometric data analysis [9,71]. Numerous articles have been recorded on the subject; thus, it is essential to utilize a credible search engine. Web of Science and Scopus are two extremely precise databases that are ideally fit for this objective [72,73]. Scopus, which comes highly recommended by academics, was used to collect bibliographic information for this study on geopolymers. As of May 2022, a Scopus search for the term "geopolymers" yielded 10,115 results. Numerous filter settings were utilized to eliminate unnecessary papers. Figure 2 depicts a complete flowchart of the data retrieval, analysis, and numerous limits/filters applied during the analysis. Additionally, other studies have reported on the same method [74][75][76]. Following the application of these filters to the Scopus database, 7468 results remained. Scopus records were stored in a Comma Separated Values (CSV) format for further assessment using the relevant software. VOSviewer (version 1.6.18) was utilized to construct the scientific visualization and quantitative evaluation of the obtained material. VOSviewer is a freely accessible, open-source mapping tool that is generally employed in distinct study areas and proposed by academics [77][78][79]. Consequently, the current study's objectives were met by the usage of VOSviewer. The resulting CSV file was loaded into the VOSviewer, and further evaluation was conducted while maintaining data consistency and reliability. During the scientometric analysis, the publishing outlets, the most frequently occurring keywords, the researchers with the highest number of published articles and citations, documents that received the most citations, and the state's involvement were evaluated. The multiple features, their interrelationships, and co-occurrences were illustrated via maps, and their quantitative data were presented in tables. The color of an item in a map is determined by the cluster to which the item belongs. In addition, for the density visualization, there are various color options such as viridis, plasma, and rainbow, and this study used the rainbow option for density mapping.

Subject Area and Yearly Publication of Documents
This assessment was performed using the Scopus analyzer to identify the most pertinent study fields. As seen in Figure 3, Materials Science, Engineering, and Environmental Science were determined to be the top three document-generating disciplines, with about 33%, 32%, and 8% of documents, respectively, contributing a total of 73% of documents. In addition, the Scopus database was analyzed for the kind of publications containing the documents on the subject research area (Figure 4). Based on this assessment, journal papers, conference articles, journal reviews, and conference reviews comprised around 76%, 18%, 4%, and 2% of all data, respectively. Figure 5 depicts the annual development of articles published in the current study field from 1983 to May 2022, since the Step # Results and discussions 5 Figure 2. Sequence of the review's approach signifying various options selected and limits applied.

Subject Area and Yearly Publication of Documents
This assessment was performed using the Scopus analyzer to identify the most pertinent study fields. As seen in Figure 3, Materials Science, Engineering, and Environmental Science were determined to be the top three document-generating disciplines, with about 33%, 32%, and 8% of documents, respectively, contributing a total of 73% of documents. In addition, the Scopus database was analyzed for the kind of publications containing the documents on the subject research area (Figure 4). Based on this assessment, journal papers, conference articles, journal reviews, and conference reviews comprised around 76%, 18%, 4%, and 2% of all data, respectively. It is fascinating to see that the researchers are focusing their attention on the use of alternative binders for construction materials.

Bibliographic Coupling of Publication Sources
The evaluation of publication outlets (journals) was performed on the bibliographic data employing the VOSviewer tool. A minimum of 30 papers per source was stipulated, and 28 of the 753 sources satisfied this requirement. Table 1

Bibliographic Coupling of Publication Sources
The evaluation of publication outlets (journals) was performed on the bibliographic data employing the VOSviewer tool. A minimum of 30 papers per source was stipulated, and 28 of the 753 sources satisfied this requirement. Table 1 displays the publishing outlets that released at least 30 publications on geopolymers research up to May 2022, along with the number of citations received within that time frame. "Construction and building materials (CONBUILDMAT)", "Ceramics international", and "Journal of cleaner production" were found to be the top publication journals with 770, 221, and 151 papers, respectively. Furthermore, the top three sources based on the number of citations received up to May 2022 are "CONBUILDMAT" with 34,289; "Cement and concrete research" receiving 12,256; and "Journal of materials science" with 9547 citations. Exceptionally, this examination would provide a groundwork for forthcoming scientometric evaluations in the research for geopolymers. Additionally, previous conventional review studies were unable of producing systematic graphs. Figure 6 depicts a visualization of sources publishing at least 30 articles. The frame dimension in Figure 6a is related to the outlet's influence on the present study field based on document count; a bigger frame size indicates a greater impact. As an illustration, "CONBUILDMAT" has a larger frame than the others, indicating that it is a journal of great significance in the present research field. Six groups/clusters were formed, characterized by a distinct color on the map (blue, red, purple, yellow, cyan, and green). Groups/clusters are developed based on the extent of the research outlet or the frequency with which they are co-cited in comparable articles [80]. The VOSviewer grouped journals according to their co-citation tendencies in published articles. For example, the red cluster comprises nine journals that have been co-cited many times in the same work. In addition, the links between closely located frames (sources) in a group/cluster are greater than those between widely spread. For example, "CONBUIDMAT" correlates more strongly with "Case studies in construction materials" than with "Materials" or "Journal of building engineering". As seen in Figure 7b, various shades correspond to varying density concentrations for a journal. Red has the highest density concentration, followed by yellow, green, and blue. "CONBUILDMAT" has a red shade implying its higher contribution to the research of geopolymers.

Co-Occurrence of Keywords
Keywords are significant in research since they distinguish and emphasize the basic subject of the study domain [81]. The minimum repetition requirement for a keyword was kept at 20, and 108 of the 7727 keywords were preserved. Table 2 records the leading 30 keywords most frequently used in published works on the subject. The five most oftenoccurring terms in the topic study field are geopolymer, fly ash, compressive strength, geopolymer concrete, and geopolymers. According to the keyword analysis, geopolymer has mostly been investigated to manufacture a sustainable construction material and is mostly researched to be produced from waste materials, such as fly ash, slag, silica fume, rice husk ash, etc. Figure 7 illustrates a systematic graph of keywords based on co-occurrences, connections, and density proportional to their occurrence frequency. A keyword's frame size in Figure 7a signifies its frequency, while its position suggests its co-occurrence in articles. In addition, the graph expresses that the top keywords have larger frames than the rest, signifying that these are essential keywords for a real investigation in the research of geopolymers. The graph highlights clusters in a manner that shows their co-occurrence in a variety of published documents. The color-encoded grouping is determined by the co-occurrence of several keywords in publications. Nine clusters are represented by diverse shades in Figure 7a. As observed in Figure 7b, distinct colors represent differing keyword density concentrations. The shades red, yellow, green, and blue are arranged according to their density strengths, with red representing the highest density concentration and blue representing the lowest. Geopolymer, fly ash, compressive strength, and other prominent keywords display red or yellow signals indicating a greater density of occurrences. This finding will help ambitious researchers select keywords that ease the discovery of published papers on a specific topic.

Authors' Coauthorship
Citations show a scientist's impact in a particular field of research [82]. The lowest paper threshold for a researcher was decided at 25, and 55 out of 9855 researchers satisfied this requirement. The authors with the most articles and citations in the field of geopolymers, as assessed from the bibliographic data using VOSviewer, are included in Table 3.
Each author's average citations were determined by dividing the total citations by the total number of articles. It is complicated to assess the effectiveness of a scientist when all parameters, such as the quantity of documents, overall citations, and average citations, are taken into account. Alternatively, the researcher's ranking will be evaluated separately for each component, i.e., the number of documents, the number of overall citations, and the average number of citations. The analysis revealed that Van Deventer J.S.J. and Chindaprasirt P.  Figure 8 depicts the association between writers with at least 25 publications and the most notable authors. Figure 8a depicts the scientific mapping of scholars who have contributed at least 25 papers to the current field of study. Figure 8b depicts the largest group of related writers based on citations, which consists of 44 of the 55 authors. This investigation indicated that the majority of researchers working on geopolymers are linked via citations.

Bibliographic Coupling of Documents
The number of citations a publication receives signifies its impact in a certain research domain. In their respective study domains, papers having a large number of citations are regarded as pioneering. The lowest number of citations for a document was kept at 200,

Bibliographic Coupling of Documents
The number of citations a publication receives signifies its impact in a certain research domain. In their respective study domains, papers having a large number of citations are regarded as pioneering. The lowest number of citations for a document was kept at 200, and 131 out of 7468 papers met this threshold. In Table 4, the top five articles in the field of geopolymers based on citations are included, along with their authors and citation counts. The work titled "Geopolymer technology: The current state of the art" by Duxson P. [53] has 2573 citations. Davidovits J. [83] and Duxson P. [84] received 2507 and 1124 citations for their articles, respectively, and were placed in the top three. However, until May 2022, just 28 papers had acquired more than 500 citations. Moreover, Figure 9 depicts the scientific visualization of connected articles on the basis of citations and the density concentration of these articles in the domain of the present study. Figure 9a displays that 128 of 131 publications were linked by citations, as determined by the VOSviewer analysis. In addition, the density mapping (Figure 9b) demonstrates the increased density concentration of the top articles based on citations.

Bibliographic Coupling of Countries
Numerous nations have presented more documents to the present research area than others, and they plan to continue their contributions. The systematic map was constructed so that readers may examine regions devoted to geopolymers research for predicting concrete properties. The minimum number of documents a nation may possess was kept 50, and 31 nations satisfied this threshold. The countries included in Table 5 have produced a minimum of 50 documents on the current topic of research. China, India, and Australia gave the greatest number of papers, with 895, 776, and 743 documents, respectively. In addition, Australia received 54,555 citations, followed by China with 22,820 citations, and the United States received 15,649 citations. Figure 10 depicts the systematic map and the density strength of countries linked by citations. Figure 10a depicts that the size of a frame is proportional to a country's impact on the topic study based on the number of articles. As seen in Figure 10b, the nations with the greatest level of participation had a greater density. The graphical depiction and quantitative record of the participating nations will assist young scientists in making scientific alliances, launching collaborative ventures, and exchanging creative methods and concepts. Scholars from states concerned with developing research on geopolymers can collaborate with experts in the field and profit from their expertise. Mclellan B.C. [52] Costs and carbon emissions for geopolymer pastes in comparison to ordinary portland cement 870 5 Turner L.K. [51] Carbon dioxide equivalent (CO 2 -e) emissions: A comparison between geopolymer and OPC cement concrete 3 Duxson P. [84] The role of inorganic polymer technology in the development of 'green concrete' 1124 4 Mclellan B.C. [52] Costs and carbon emissions for geopolymer pastes in comparison to ordinary portland cement 870 5 Turner L.K. [51] Carbon dioxide equivalent (CO2-e) emissions: A comparison between geopolymer and OPC cement concrete 862

Discussion
This systematic review performed the statistical analysis and mapping of the bibliographic data available in the research of geopolymers. Previous manual review studies lacked the capacity to completely and precisely link disparate areas of the literature. This analysis identified the sources of publications (journals) that published the most documents, the most often-used keywords in publications, the documents and researchers with the highest citations, and the nations vigorously engaged in geopolymers research. According to the assessment of keywords, geopolymer has mostly been researched to produce sustainable construction material and is mostly investigated to be produced from waste materials such as fly ash, slag, silica fume, rice husk ash, etc. In addition, the literature and their linkages based on citations were used to identify the highly committed and participating nations based on publication count. The graphical representation and quantitative analysis of the participating countries and researchers will help young scientists in forming scientific partnerships, establishing joint ventures, and sharing advanced methods and concepts. Scholars from countries concerned with expanding the research on the application of geopolymers can collaborate with professionals in the discipline and benefit from their expertise. After assessing keywords in the subject topic using the scientometric analysis method and reviewing the most relevant literature, this study highlighted and discussed the present-state applications of geopolymer concrete, and the limitations associated with the production and use of geopolymer composites and possible solutions in the following subsections.

Applications of Geopolymer Concrete
Geopolymer technology has a significant possibility for reusing waste materials, thus solving the problem of environmental pollution caused by the disposal of these items in landfills. In the domains of fire-resilient and asbestos-free materials, high-tech materials, new ceramics, and harmful waste-stabilization matrices, geopolymer-based products have been utilized [85]. Cement and concrete are essential to modern development because they facilitate the creation of the infrastructure required for a superior level of livelihood.
It is believed that geopolymers provide a road for more commercialization, large-scale production, and a basis for rapid interpretation of research pertaining to comments regarding the eco-responsiveness of building materials. In addition, an evaluation of the optimal composition and transformation conditions must be conducted with respect to the geography and supply chains of raw materials [86].
The use of geopolymer concrete in various structures such as precast bridge decks, retaining walls, road pavements, aircraft pavements, water tanks, boundary blocks, and precast beams has increased in Australia [87]. Other studies have established the use of geopolymer concrete as a structural element, justifying its current usage in Australia and parts of Europe. Recognition and implementation of geopolymer concrete are achieving traction. In 2016, the government of Japan established a committee to study the current usage of geopolymer concrete structural elements in different global locations. The International Union of Laboratories and Experts in Construction Materials, Systems, and Structures (RILEM, after its French acronym) has tasked a committee with recognizing and validating methods for evaluating the durability of geopolymer composites. The study comprised 15 laboratories from around the world. Since geopolymer composites do not have a lengthy track record of durability [88,89], it is crucial to establish these test and validation procedures. In particular places worldwide, the application of geopolymer concrete is increasing quickly, whilst in others, it is advancing gradually [90]. Alkali-activated binders might be a helpful ingredient of eco-friendly construction materials if they are created effectively, their carbon impact is considered, and local resources are utilized. Because of the necessity for exact mix design and curing, technical issues connected with certain kinds of applications, and supply chain restrictions, it is doubtful that geopolymer concrete will be able to replace conventional concrete in a like-for-like manner. Furthermore, even if regulatory authorities understood the results of durability experiments on geopolymer concrete, the procurement of raw ingredients for its production would be a barrier. In addition, since the supply chain for cement-based composites depends on OPC, controlling and product-assurance obstacles must be overwhelmed to permit the usage and approval of geopolymer composites. Moreover, it is essential to emphasize that users, academia, and regulatory bodies recognize the commercialization potential of geopolymer composites [91,92]. Manufacturing geopolymer tiles for increased temperatures is another possible application of geopolymer composites [93][94][95].

Limitations and Potential Remedies
Since geopolymers eliminate the need for OPC in building materials, the introduction of geopolymer composites has created new opportunities for sustainability in the building industry. However, there are a number of constraints that limit their future applicability.

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Geopolymer composites need steam/heat curing for strength enhancement [96,97], which is challenging to apply to structural elements on-site. Nevertheless, enclosing the structural elements with films and creating a humid atmosphere might be an alternate curing method for geopolymer composites [98,99]. • Sodium-silicate deficiency is a further significant factor limiting geopolymer production. The environmental efficacy and financial advantage of geopolymers are highly dependent on the amount of alkali-activated material employed [23]. Geopolymer composites are also costly because of the high cost of activating solutions [100]. Utilizing rice husk ash as a silicon source in the production of sodium-silicate solution might reduce the requirement for sodium carbonate and quartz sand [101], both of which produce greenhouse gases during the production. Sodium silicate produced from rice husk ash and waste glass is an outstanding activator for the production of metakaolin-based geopolymers [102]. Utilizing glass-polishing waste as an activator may also be a cost-efficient option [103].

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Another factor that impacts the advancement of geopolymer composites is efflorescence. Salt growth in surface alkali-activated cement has been seen in several industrial applications and laboratory experiments, where it has been described as efflorescence. In silicon-rich settings, high concentrations of alkali activators can induce substantial efflorescence. The level of efflorescence decreased as the content of alumina increased [104]. The occurrence of geopolymer efflorescence decreases as silica concentration and grain size increase [105,106]. The use of nanosilica in geopolymer composites based on metakaolin is beneficial. Nanosilica decreased efflorescence by consuming alkali ions in excess from the pore solution, therefore forming an amorphous gel phase [107].

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The use of alkaline activators in greater concentrations may hasten ASR, hence restricting the usefulness of geopolymer composites. According to the researchers, ASR expansion was more easily generated in high-calcium and mixed systems than in lowcalcium alkali-activated cement systems. Activators of varying kinds and dilutions can also induce ASR. Certain admixtures may lead to the development of ASR [108]. Therefore, further study on ASR is necessary to determine the crucial factors that influence its incidence.

Conclusions
This study's purpose was to undertake a scientometric assessment of the available literature on geopolymers to evaluate different metrics. The database Scopus was searched for 7468 related articles, and the records were evaluated employing the VOSviewer application. The following conclusions were obtained from this study: • An evaluation of publication journals, including articles on geopolymers research, revealed that "CONBUILDMAT", "Ceramics international", and "Journal of cleaner production" are the top three sources, with 770, 221, and 151 publications, respectively. In terms of total citations, the top three publishing sources are "CONBULDMAT" with 34,289, "Cement and concrete research" with 12,256, and "Journal of materials science" with 9547 citations. • Assessment of keywords on the topic research field reveals that geopolymer, fly ash, compressive strength, geopolymer concrete, and geopolymers are the five most often occurring terms. The keyword analysis found that geopolymer has mostly been researched to produce sustainable construction material and is mostly investigated to be produced from waste materials, such as fly ash, slag, silica fume, rice husk ash, etc. Since geopolymers require source materials with greater aluminosilicate concentrations, which are prevalent in various waste materials such as fly ash, slag, rice husk ash, etc., recycling these materials to create geopolymers would minimize environmental pollution.

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The large-scale applications of geopolymer concrete in the building sector are limited due to several restraints such as curing regime, deficiency and cost of activator solution, efflorescence, and ASR. Adopting potential remedies as discussed in this study might help increase the acceptance of geopolymer concrete in construction. However, further in-depth investigations are necessary on these solutions for the large-scale applicability of geopolymers.